1. Field of the Invention
The present invention relates to an electrochromic display element in which an electrochromic compound exhibits high-speed response of color developing or erasing as well as image blur is reduced.
2. Description of the Related Art
In recent years, development has extensively been made on electronic paper as a medium in place of paper. Electronic paper is a display device used like paper, and thus needs to meet requirements different from those for conventional display devices such as CRTs and liquid crystal displays. For example, electronic paper is required to have high white reflectance, high contrast ratio, high-definition display performance and display memory effect as well as to be thin, light, inexpensive and able to drive at even low voltages. Of these, white reflectance and contrast ratio that are comparable to those of paper are highly required, since these requirements influence the display quality obtained.
Display devices for use in electronic paper include those using reflective liquid crystals, those using electrophoresis and those using toner migration.
Any of these display devices, however, has much difficulty in performing multi-color display while ensuring white reflectance and contrast ratio. In general, a color filter is provided for performing multi-color display. In this case, the color filter, whose pixels are each segmented into three regions of red (R), green (G) and blue (B), absorbs light to problematically reduce reflectance of the display device.
In order to solve such problems, electrochromic display elements with no color filter have been proposed. The electrochromic display elements develop or erase color using an electrochromic compound which reversibly changes in color through reversible oxidoreduction reaction upon application of voltage.
The electrochromic display elements, however, have problematically slow response speed of color developing or erasing, since they develop or erase color utilizing oxidoreduction reaction.
In order to solve such problems, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-510590 proposes a method in which an electrochromic compound is fixed in the vicinity of an electrode to increase response speed of color developing or erasing. According to this proposal, although the time required from color developing to color erasing is conventionally about 10 sec, the color developing time from colorless to blue and the color erasing time from blue to colorless are both shortened to be about 1 sec.
However, demand has arisen for further improvement of response speed of color developing or erasing.
Meanwhile, the electrochromic display elements can develop various colors depending on the structure of the electrochromic compound, and become a promising multi-color display device. For example, JP-A No. 2003-121883 proposes a multi-color display device containing, in a stacked state, several kinds of electrochromic compound fine particles that develop color at different voltages.
However, by controlling the voltage, this multi-color display device can develop one color but cannot develop several colors simultaneously, since the electrochromic compounds stacked in the multi-color display device are compounds that develop different colors at different voltages.
Also, JP-A No. 2006-106669 proposes a display device containing electrochromic layers on an electrode, which are made of several different electrochromic compounds having different threshold voltages and charge quantities for color developing. This display device develops multi colors utilizing differences between the voltages or currents required for color developing.
The display device according to this proposal can develop several colors simultaneously, but requires complicate control of the voltage or current to selectively develop any color, since the display device has several kinds of electrochromic compounds that develop different colors.
In order to solve such problems, for example, JP-A No. 2010-033016 proposes an electrochromic display element containing a display substrate, a plurality of display electrodes provided on the display substrate, and a plurality of electrochromic layers provided correspondingly to the display electrodes.
However, use of an electrolyte prepared by dissolving a supporting electrolyte in a solvent, as described in prior arts, causes charges to be diffused in the electrolyte from the electrochromic compound developing color, leading to problematic image blur; i.e., bleeding due to spread of color-developing portions which occurs over time when the nearby electrochromic compounds not developing color also cause color-developing reaction. Also, use of a solid electrolyte such as a polymer electrolyte, as described in prior arts, prevents image blur to some extent but retards response of color developing or erasing.
Therefore, at present, keen demand has arisen for development for an electrochromic display element in which an electrochromic compound exhibits high-speed response of color developing or erasing as well as image blur is reduced.